The present invention relates to a circuit board made of resin with pins and more particularly to a circuit board made of resin with pins comprising interlaminar insulating layers formed by a resin material and pins (input and output terminals) soldered thereto on a number of pin bonding portions (electrodes) formed on the main surface of the circuit board as in a PGA (pin grid array) type package circuit board having electronic parts such as semiconductor integrated circuit element (IC) mounted and encapsulated thereon.
A PGA type circuit board made of resin (hereinafter simply referred to as “board”) has a number of pad-shaped pin bonding portions (electrodes) provided on one main surface thereof for mounting electronic parts such as LSI and IC chip and a number of pins provided on the other main surface thereof for inserting in sockets provided on mother board, etc. As such a pin there is normally used, e.g., nail-shaped flat pin comprising a rod portion and a tabular large diameter portion (having a greater diameter than the rod portion) formed on one end thereof. This flat pin is bonded to the circuit board with a solder layer formed by soldering with its large diameter portion disposed opposed to the pin bonding portion of the board.
In the aforementioned type of circuit board made of resin with pins, a tendency is given that more circuit boards made of resin have smaller size and more connecting terminals (pins) to attain higher integration and density of electronic parts such as LSI, IC chip and chip capacitor to be mounted. Therefore, the size of even the flat pin mounted on such a circuit board made of resin is reduced by reducing the diameter of the rod portion thereof.    [Patent Reference 1]            JP-A-2001-267451            [Patent Reference 2]            JP-A-2001-217341            [Patent Reference 3]            JP-A-2001-358277        
Referring to the reduction of size of flat pins provided on the aforementioned circuit board made of resin with pins, it is recently numerically required that the diameter of the rod portion be about 0.30 mm, particularly not greater than 0.35 mm (e.g., from not smaller than 0.25 mm to not greater than 0.35 mm). In the case where the diameter of the rod portion of the flat pin is reduced, it is necessary that the size of the large diameter portion of the flat pin be optimized depending on the diameter of the rod portion thus reduced. When this optimization is not made, a sufficient bonding strength of the large diameter portion of the flat pin with the pin bonding portion on the circuit board made of resin with pins cannot be secured. For example, when an excessive unexpected external force occurs in the axial direction or direction oblique thereto during the insertion of the pins in the sockets on the mother board, exfoliation can occur in the vicinity of the border of the solder layer for bonding the flat pin with the pin bonding portion or the border of the large diameter portion of the flat pin with the solder layer. This naturally makes it impossible for the circuit board made of resin with pins to maintain its quality requirements such as electrical characteristics.